The miniaturization of electronic devices has created a demand for small but powerful electrochemical cells. Cells that utilize an alkaline electrolyte are known to provide high energy density per unit volume, and are therefore well suited for applications in miniature electronic devices such as hearing aids, watches and calculators. However, alkaline electrolytes, such as aqueous potassium hydroxide and sodium hydroxide solutions, have an affinity for wetting metal surfaces and are known to creep through the sealed metal interface of an electrochemical cell. Leakage in this manner can deplete the electrolyte solution from the cell and can also cause a corrosive deposit on the surface of the cell that detracts from the cell's appearance and marketability. These corrosive salts may also damage the device in which the cell is housed. Typical cell systems where this problem is encountered include silver oxide-zinc cells, nickel-cadmium cells, air depolarized cells, and alkaline manganese dioxide cells.
Many liquid electrolytes used in galvanic cells will form a corrosive salt deposit on the exterior surface of the cells in which the electrolyte is used if the electrolyte leaks out of the cell. Such a corrosive deposit detracts from the appearance and marketability of the cell. These corrosive deposits may also damage the electronic device in which the cell is housed and short the cell. Therefore such galvanic cells are sealed to prevent electrolyte leakage.
In the prior art it has been a conventional practice to incorporate insulating gaskets between the cell container and cover so as to provide a seal for the cell. Generally, the gasket must be made of a material inert to the electrolyte contained in the cell and the cell environment. In addition, it had to be flexible and resistant to cold flow under pressure of the seal and maintain these characteristics so as to insure a proper seal during long periods of storage. Materials such as nylon, polypropylene, ethylene-tetrafluoroethylene copolymer and high density polyethylene have been found to be suitable as gasket materials for most applications. Typically, the insulating gasket is in the form of a "J" shaped configuration in which the extended wall of the cover is inserted so that upon being radically squeezed, the bottom portion of the gasket forms a "U" shaped seal for the bottom portion of the wall of the container. To better insure a good seal, a sealant is generally deposited in the "J" shaped seal so that upon insertion of the cover into the gasket, the edge of the extended wall of the cover will seat in the sealant and then upon the application of a radical squeeze, the gasket will be compressed against the bottom portion of the extended cover wall. The sealant will thereby act as a further seal for the cover-gasket assembly. In addition, the sealant can also function as a gap filler to compensate for variances in the manufacture of the gasket and cover components. It is possible that deviation from the ideal component sizes of the gasket and cover could result in the active battery materials and/or electrolyte being trapped between the edge of the wall of the cover and the sealant. This could result in cell leakage. Consequently, since component parts of the gasket-cover assembly are difficult to manufacture to exact specifications and it is difficult to apply the sealant with precision to the gasket on a continuous operational basis, there is always the possibility that the assembled cells will leak. This is particularly true of small miniature cells.
It is an object of the present invention to provide a process for producing a good gasket-cover seal assembly for electrochemical cells.
It is another object of the present invention to provide a process using ultrasonic means for producing a good gasket-cover seal assembly for electrochemical cells, such as alkaline cells.
It is another object of the present invention to provide an efficient and cost effective process that uses ultrasonic means for producing a gasket-cover seal assembly for alkaline cells.
The foregoing and additional objects of the present invention will become more fully apparent from the following description and accompanying drawings.